Contention-based networks, including local area networks such as Ethernet® networks, were originally designed with bursty data transmission in mind. More recently, such networks have been looked toward to support real-time applications such as voice and video. However, using contention-based networks to carry real-time traffic may be problematic because of such networks' inability to guarantee a maximum, or bounded, delay service and thus their inability to guarantee a desired level of service quality for real-time applications.
In order to address this issue, techniques have been proposed which give a certain degree of priority to real-time traffic. One approach is a pre-emptive approach wherein a station desiring to transmit a real-time packet simply comes on to the network's communications medium, e.g., coaxial cable, and transmits a fixed-length preamble. This causes a collision with any then existing traffic. Pursuant to conventional protocol schemes, the station that was transmitting the existing traffic ceases transmitting upon detecting that the collision occurred. At that point the real-time station which caused to collision simply continues with the transmission of its real-time packet. While serving to provide immediate access for the real-time station, this approach has the disadvantage that any packet that was being transmitted at the time of the collision will have to be retransmitted, thereby reducing the overall efficiency of the network. Moreover, the probability of such a collision increases with increasing network load, thereby increasing the number of collisions and further reducing efficiency at higher network loads.
A more efficient approach is that described for example, in I. Chlamtac, “An Ethernet Compatible Protocol for Real-Time Voice/Data Integration,” Computer Networks and ISDN Systems, 10 (1985), Elsevier Science Publishers B.V. (North-Holland), pp. 81-96, in which the various currently active real-time stations establish an order of transmission among themselves and use, for example, the notion of a succession, or chain, wherein the real-time stations transmit one after the other in a way which does not allow the medium to be perceived by the non-real-time stations as being idle. It is only after all the real-time stations have had an opportunity to transmit packets they then have ready that the medium again becomes idle, and thus available to any station then having data to send. See also, for example, Szabo, “An Ethernet compatible protocol to support real time traffic and multimedia applications,” Computer Networks and ISDN Systems, Vol. 29, page 335-42, 1997.
Disadvantageously, as more and more real-time stations are added to the chain, the average delay suffered by non-real-time traffic increases due to the reduced time available for such traffic; an increase in the duration of the continuous period over which the non-real-time stations are precluded from getting access to the medium; and an increase in the probability of collisions involving non-real-time stations once the stations of the chain have all finished transmitting. Indeed, given a desired maximum such delay, and especially at high network loads, an upper limit exists on the number of real-time stations that can be allowed to be simultaneously active.